1. Field of the Invention
The present invention relates to a polishing apparatus and method, and more particularly to an apparatus and method for polishing a workpiece such as a semiconductor wafer to a flat mirror finish.
1. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 xcexcm wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small. However, conventional apparatuses for planarizing semiconductor wafers such as self-planarizing CVD apparatus or etching apparatus fail to produce completely flat surfaces on semiconductor wafers. Recently, it has been attempted to planarize semiconductor wafers with a polishing apparatus which is expected to achieve complete planarization of the semiconductor wafers with greater ease than the above conventional apparatuses. Such a process is called Chemical Mechanical Polishing (CMP) in which the semiconductor wafers are chemically and mechanically polished while supplying an abrasive liquid comprising abrasive grains and chemical solution such as alkaline solution.
Conventionally, such a polishing apparatus has a turntable and a top ring which rotate at respective individual speeds. A polishing cloth is attached to the upper surface of the turntable. A semiconductor wafer to be polished is placed on the polishing cloth and clamped between the top ring and the turntable. An abrasive liquid containing abrasive grains (or material) is supplied onto the polishing cloth and retained on the polishing cloth. During operation, the top ring exerts a certain pressure on the turntable, and the surface of the semiconductor wafer held against the polishing cloth is therefore polished by a combination of chemical polishing and mechanical polishing to a flat mirror finish while the top ring and the turntable are rotated.
In the polishing apparatus for polishing the surface of a semiconductor wafer, especially a device pattern on the upper surface of a semiconductor wafer, to a planar finish a polishing cloth attached to a turntable made of nonwoven fabric has heretofore been employed.
Higher levels of integration achieved in recent years for ICs and LSI circuits demand smaller steps or surface irregularities on the polished surface of the semiconductor wafer. In order to meet such a demand, it has been proposed to employ a polishing cloth made of a hard material such as polyurethane foam.
After the semiconductor wafers are contacted with the polishing cloth and polished by rotating the turntable and the top ring which holds the semiconductor wafer, the polishing capability of the polishing cloth is gradually deteriorated due to a deposit of abrasive grains and groundoff particles of the semiconductor material, and due to changes in the characteristics of the polishing cloth. Therefore, if the same polishing cloth is used to repeatedly polish semiconductor wafers, the polishing rate of the polishing apparatus is lowered, and the polished semiconductor wafers tend to suffer polishing irregularities. Therefore, it has been customary to condition the polishing cloth according to a process called xe2x80x9cdressingxe2x80x9d for recovering the surface of the polishing cloth before, or after, or during polishing.
One way of dressing a polishing cloth made of a hard material such as polyurethane foam is to use a diamond dresser. The diamond dresser is advantageous in that it is effective to recover the desired polishing surface of the polishing cloth and does not cause a lowering of the polishing rate.
When the diamond dresser dresses the polishing surface of the polishing cloth, it scrapes a thin layer off the polishing cloth. Since the diamond dresser dresses only a limited inner area of the polishing cloth which is used for polishing semiconductor wafers, and slightly marginal areas located inside and outside of the limited area, the polishing surface of the polishing cloth becomes irregular, i.e., loses its planarity, after it has been dressed many times. As a result, an annular step of certain width is formed on the polishing surface along an outer circumferential edge of the polishing cloth and has an upper surface higher than the upper surface of the dressed inner area.
After a semiconductor wafer is polished with the polishing cloth thus dressed, it is necessary to remove the semiconductor wafer from the polishing cloth. However, if the top ring holding the semiconductor wafer is raised to remove the semiconductor wafer from the polishing cloth, the surface tension between the polishing cloth and the semiconductor wafer is large, and there are some cases that only the top ring is raised and the semiconductor wafer adheres to the polishing cloth to be left on the polishing cloth.
It is therefore an object of the present invention to provide a polishing apparatus and method which: have an overhanging function to remove a workpiece such as a semiconductor wafer from a polishing surface of a polishing cloth after a polished surface of the workpiece is partly exposed beyond the outer circumferential edge of the polishing cloth in overhanging relation thereto; can prevent the workpiece from cracking when the workpiece is in the over state; and can also prevent an unexpected remaining mark from being formed on the polished surface of the workpiece.
According to one aspect of the present invention, there is provided an apparatus for polishing a surface of a workpiece, comprising: a turntable having a polishing surface thereon; a top ring for holding a workpiece and pressing the workpiece against the polishing surface under a given pressure to polish the workpiece; a mechanism for moving the top ring in a horizontal plane while the workpiece is in contact with the polishing surface to project a part of polished surface of the workpiece from the polishing surface after polishing, and then raising the top ring holding the workpiece to remove the workpiece from the polishing surface; and a dresser for dressing the polishing surface. The dresser dresses the polishing surface from an inner side to an outer circumferential edge of the polishing surface so that a step is not formed from an area located at the inner side of the polishing surface and used for polishing the workpiece to the outer circumferential edge of the polishing surface.
According to another aspect of the present invention, there is also provided a method for polishing a surface of a workpiece, comprising: dressing a polishing surface of a turntable from an inner side to an outer circumferential edge of the polishing surface so that a step is not formed from an area located at the inner side of the polishing surface and used for polishing the workpiece to the outer circumferential edge of the polishing surface; holding the workpiece and pressing the workpiece against the polishing surface under a given pressure by a top ring; and moving the top ring in a horizontal plane while the workpiece is in contact with the polishing surface to project a part of polished surface of the workpiece from the polishing surface after polishing, and then arising the top ring holding the workpiece to remove the workpiece from the polishing surface.
According to the present invention, when the polishing surface on the turntable is dressed, it is dressed to its outer circumferential edge by the dresser to thereby minimize or eliminate formation of a step. Therefore, when the polished workpiece is horizontally displaced to a position where a polished surface of the workpiece is partly exposed beyond the outer circumferential edge of the polishing surface in overhanging relation thereto, the workpiece is prevented from cracking. Further, the workpiece is free of any swirling marks on its polished surface.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.